1. Field of the Invention
The present invention relates to a method for producing glycidol by decarboxylation of glycerol carbonate.
2. Description of the Related Art
In recent years, there has been an increasing demand for and interest in biodiesel, which is synthesized from vegetable oils or animal fats, as a new renewable energy source. Biodiesel is produced by reaction of fatty acids with alcohols. Glycerol as a by-product accounts for about 10% by weight of the total biodiesel production. The price of glycerol is declining due to oversupply. One approach to convert cheap glycerol to high value-added products is thermal decomposition of glycerol carbonate as a glycerol derivative to synthesize glycidol. Glycidol is currently used in various applications, including cleaning agents in the petroleum industry, drug delivery materials, and raw materials for polymers. Glycidol is industrially produced by oxidation of allyl alcohol with hydrogen peroxide in the presence of a catalyst. However, this method involves a complicated process to purify glycidol despite its high production yield. Another disadvantage of the method is that a tungsten oxide compound as the catalyst tends to decompose during the reaction, resulting in an increase in the production cost of glycidol.
FIG. 1 shows a novel synthetic method for glycidol based on the thermal decomposition of glycerol carbonate. According to this method, the reaction proceeds in the presence of a metal salt catalyst. As a result of the reaction, glycidol as a product is produced along with carbon dioxide as a by-product. The carbon dioxide is reacted with ammonia to produce urea, which is then reacted with glycerol to synthesize glycerol carbonate as a raw material for glycidol. The method is associated with the production of glycidol by decarboxylation of glycerol carbonate derived from glycerol and is worthy of development from an economic and environmental viewpoint due to the use of the biomaterial-based raw material.
In this connection, U.S. Pat. No. 2,856,413 and Japanese Patent Publication No. Hei 6-157906 disclose methods for producing glycidol from glycerol carbonate using metal salts, such as Na3PO4, CaCO3 and Na2SO4 as catalysts. Although these patent publications propose the use of various metal salt catalysts, they fail to mention approaches to inhibit side reactions.
Further, Japanese Patent Publication No. 2009-137938 and U.S. Pat. No. 6,316,641 disclose methods for synthesizing glycidol in high yield. According to these methods, the reaction may proceed in an active hydrogen-free solvent, such as an ether-based solvent, an aromatic hydrocarbon-based solvent or a saturated hydrocarbon-based solvent, in the presence of Na2SO4 or zeolite A as a catalyst, or the reaction may proceed using a thin-film reactor. The methods are effective in inhibiting side reactions to obtain glycidol in high yield, but the ability of the catalyst to inhibit side reactions is limited.
Similarly to the methods for producing glycidol from glycerol carbonate, methods for producing glycidol from glycerol and ethylene carbonate, propylene carbonate or butylene carbonate are disclosed in U.S. Pat. No. 2,636,040. According to these methods, glycidol is produced by gradually heating glycerol and an alkylene carbonate from 145° C. to 155-240° C. while decreasing the pressure from 75 mmHg to 10-15 mmHg in a distilled column to obtain glycerol carbonate as an intermediate, and distilling the intermediate at 11 mmHg and 80° C. However, a disadvantage of the methods is that reverse reactions are likely to occur in the course of obtaining the intermediate.